Oklahoma, Alabama, Joplin: Why we're seeing so many tornadoes and superstorms
The superfunnels that hit Tuscaloosa, Ala., in late April and Joplin, Mo., on Sunday, are generated by storm systems whose journeys across the country are slowed by a roadblocked jet stream.
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At this atmospheric crossroads, intense changes in speed and direction with height can set the entire, broad-scale "supercell" thunderstorm rotating, explains Mr. Imy.Skip to next paragraph
In Pictures Missouri tornado
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Recent research has shown that a final necessary ingredient is additional shear within the first 2,000 to 3,000 feet above the surface, Imy adds.
This shear manifests itself as a vortex, oriented like a paper-towel tube sitting on a table. If the updraft in a thunderstorm is strong enough, it can in effect stand the vortex on its end. This upended vortex can ultimately spawn a funnel cloud.
The stronger the shear, the stronger any resulting tornado is likely to be – and the longer its track on the ground.
Supercell storms, like the one that struck Joplin, tend to generate the most powerful twisters.
With the storms that spawned the tornadoes across the eastern US in late April, as well as with the system that is still generating severe weather in the US heartland, "we had very strong shear," Imy says.
Superstorm-filled spring: Evidence of global warming?
The record-breaking burst of tornadoes at the end of April, as well as this week's events, have sparked speculation that global warming could be playing a role.
If that's the case, global warming's fingerprint is still too weak to pick out of the data, according to a preliminary analysis by Martin Hoerling, a researcher with the the National Oceanic and Atmospheric Administration's Earth Systems Research Laboratory in Boulder, Colo.
Tornadoes – even mile-wide twisters – happen on spatial scales far too small for climate models to deal with. But some scientists have started looking at the broader atmospheric conditions that can trigger the formation of thunderstorms capable of generating twisters.
In a draft analysis posted earlier this month, Dr. Hoerling analyzed data for the month of April for the lower Mississippi Valley between 1979 to 2010. At least for each April in his initial sample, the analysis shows no detectable trend in conditions that would generate tornadic thunderstorms.
Still, he writes, the lack of detection now "does not exclude that a future change" in the atmospheric conditions he analyzed "may occur" as humans continue to pump greenhouse gases into the atmosphere.
Another analysis by atmospheric scientist Roy Spencer at the University of Alabama in Huntsville suggests that over the past 60 years, the number of "intense" tornadoes in the US from March through August – twisters rated in the three highest categories of tornado intensity – have declined, even as global average temperatures have risen.